In many internal combustion engines the engine cylinder intake and exhaust valves may be opened and closed by fixed profile cams in the engine, and more specifically by one or more fixed lobes which may be an integral part of each of the cams. The use of fixed profile cams makes it difficult to adjust the timings and/or amounts of engine valve lift to optimize valve opening times and lift for various engine operating conditions, such as different engine speeds.
One method of adjusting valve timing and lift, given a fixed cam profile, has been to incorporate a "lost motion" device in the valve train linkage between the valve and the cam. Lost motion is the term applied to a class of technical solutions for modifying the valve motion proscribed by a cam profile with a variable length mechanical, hydraulic, or other linkage means. In a lost motion system, a cam lobe may provide the "maximum" (longest dwell and greatest lift) motion needed over a full range of engine operating conditions. A variable length system may then be included in the valve train linkage, intermediate of the valve to be opened and the cam providing the maximum motion, to subtract or lose part or all of the motion imparted by the cam to the valve.
This variable length system (or lost motion system) may, when expanded fully, transmits all of the cam motion to the valve, and when contracted fully, transmit none or a minimum amount of the cam motion to the valve. An example of such a system is provided in U.S. Pat. No. 5,537,976 to Hu and U.S. Pat. No. 5,680,841, also to Hu, which are assigned to the same assignee as the present application, and which are incorporated herein by reference.
In the lost motion system of U.S. Pat. No. 5,680,841, an engine cam may actuate a master piston which displaces fluid from its hydraulic chamber into a hydraulic chamber of a slave piston. The slave piston in turn acts on the engine valve to open it. The lost motion system may be a solenoid valve and a check valve in communication with the hydraulic circuit including the chambers of the master and slave pistons. The solenoid valve may be maintained in a closed position in order to retain hydraulic fluid in the circuit. As long as the solenoid valve remains closed, the slave piston and the engine valve respond directly to the motion of the master piston, which in turn displaces hydraulic fluid in direct response to the motion of a cam. When the solenoid is opened temporarily, the circuit may partially drain, and part or all of the hydraulic pressure generated by the master piston may be absorbed by the circuit rather than be applied to displace the slave piston.
In many electronically controlled valve actuation systems, there is a need to detect the motion and timing of a valve actuator so as to know the condition of the engine during operation. In some cases, knowing the phasing or timing of the event can be used to control the system and can compensate for changes in system operating conditions or other factors. In other cases, detecting the absence of valve motion allows the control system to shut off fuel injection or other valve motions for an affected cylinder so as to prevent engine damage. In the present invention, Applicants further disclose a system for detecting the motion of a valve actuator that may be used in a common rail or lost motion valve actuation system. A low-cost, on/off position sensor is used to detect whether or not a slave piston has moved, thus providing confirmation that the circuit is operational. By checking the time at which the slave piston moves to a certain distance (that at which the sensor changes state), the control module can compensate for system leads/lags versus desired timing.
In designing lost motion valve actuation systems, many different approaches have been considered. Hydromechanical systems allow for partial lost motion, while preserving mechanical valve actuation to some lesser extent than standard. These designs are somewhat complex, and experience difficult loading conditions during compression release retarding. Valve train designs employing a purely hydraulic system are flexible and conceptually simple to design, requiring only hydraulic connections between master pistons and slave pistons. For example, U.S. Pat. No. 4,278,233 to Zurner et al. discloses a hydraulic system for actuating gas-change valves in an internal combustion engine. Such systems are unlikely to achieve rapid acceptance in the conservative engine market due to their pronounced departure from conventional technology. These systems will not operate at all if oil pressure, fluid passage continuity or electrical element control is lost.
Previous lost motion systems have typically not utilized high speed mechanisms to rapidly vary the length of the lost motion system. Lost motion systems of the prior art have accordingly not been variable such that they may assume more than one length during a single cam lobe motion, or even during one cycle of the engine. By using a high speed mechanism to vary the length of the lost motion system, more precise control may be attained over valve actuation, and accordingly optimal valve actuation may be attained for a wide range of engine operating conditions.
Applicants have determined that the lost motion system of the present invention may be particularly useful in engines requiring valve actuation for both positive power and for compression release retarding and exhaust gas recirculation valve events. Typically, compression release and exhaust gas recirculation events involve much less valve lift than do positive power related valve events. Compression release and exhaust gas recirculation events may, however, require very high pressures and temperatures to occur in the engine. Accordingly, if left uncontrolled (which may occur with the failure of a lost motion system), compression release and exhaust gas recirculation could result in pressure or temperature damage to an engine at higher operating speeds. Therefore, Applicants have determined that it may be beneficial to have a lost motion system which is capable of providing control over positive power, compression release, and exhaust gas recirculation events, and which will provide only positive power or some low level of compression release and exhaust gas recirculation valve events, should the lost motion system fail.
An example of a lost motion system used to obtain retarding and exhaust gas recirculation is provided by U.S. Pat. No. 5,146,890 to Gobert, assigned to AB Volvo, and incorporated herein by reference. Gobert discloses a method of conducting exhaust gas recirculation by placing the cylinder in communication with the exhaust system during the first part of the compression stroke and optionally also during the latter part of the inlet stroke. Gobert uses a lost motion system to enable and disable retarding and exhaust gas recirculation, but such a system is not variable within an engine cycle.
The challenge addressed by the present invention is to employ lost motion valve actuation to achieve the benefits of variable valve actuation and the flexibility of hydraulic valve train design while preserving a predictable operating mode in the event of startup or failure conditions. In the present invention, Applicants disclose embodiments directed to both a fully hydraulic valve actuation system and a hydromechanical valve actuation system with electrical control.
Applicants' method for implementing the flexible advantages of a fully hydraulic lost motion valve actuation system, while incorporating some measure of fail-safe operation, is accomplished by limiting the amount of motion which can be lost by designing the accumulator to accept less than a complete master piston stroke of working fluid.
In another embodiment of the present invention, Applicants disclose a system for valve actuation that employs a hydromechanical system with fail-safe features. It is known that internal combustion engines can be used to effect kinetic energy braking of a rolling vehicle by interrupting the engine's fuel flow, and operating the engine as an air compressor. In this mode, the rolling vehicle's kinetic energy is converted to potential energy (compressed air), and subsequently the potential energy is depleted by exhausting the compressed air into the atmosphere through the vehicle's exhaust system. Engine braking is described in detail in U.S. Pat. No. 3,220,392 to Cummins, which is incorporated herein by reference.
The effectiveness of engine compression braking can be improved further by recirculating exhaust gas into each cylinder at the time a cylinder's piston is at or near dead bottom at the beginning of the normal compression stroke. This process is commonly referred to as Exhaust Gas Recirculation or "EGR". Including EGR in a compression braking cycle will result in the introduction of a greater volume of air to a given engine cylinder. Consequently, the engine works harder compressing the denser air volume and, as a result, more kinetic energy is converted into potential energy resulting in greater engine retardation.
EGR may also be used during normal positive power operation. The benefits derived from EGR during positive power operations are: (1) increased fuel-use efficiency due to the consumption of unburned combustibles in the exhaust gas; and (2) cleaner exhaust gas emissions. Details of EGR operating modes are provided in U.S. Pat. No. 5,787,859, which is assigned to the same assignee as the present application, and which is incorporated herein by reference.
Cylinder exhaust valves open at different times during engine braking and EGR operations than during positive power operations. For engine braking, the exhaust valves open at or near top dead center at the completion of a cylinder's compression stroke. For EGR events, the exhaust valve opens at or near the aforementioned dead bottom at or near the beginning of the compression stroke. The engine's conventional valve opening system associated with positive power operations holds a cylinder's exhaust valve closed at these times. Consequently, add-on systems that augment or modify the conventional exhaust valve opening system may be applied to internal combustion engines in order to permit engine braking and EGR operating modes.
Present engine braking and EGR systems derive the time for opening each cylinder's exhaust valve from a neighboring cylinder's intake or exhaust valve opening systems. The mechanical motion of the neighboring cylinder's main event valve opening system is transmitted to the selected cylinder's exhaust valve by add-on mechanical or hydro-mechanical systems. Engine braking and EGR exhaust valve opening derived in this fashion have certain disadvantages. For example, it may not be possible to open the exhaust valve at the optimum time for EGR and brake events. Also, the add-on systems add additional weight and size to an engine. As a result, there is a need for a system which provides optimum exhaust valve timing, opening duration, and lift for EGR and braking events. A system which provides independent control of each cylinder's valve(s) would be capable of provide optimum opening profiles (timing, duration, and lift) and result in increased braking energy, and improved engine efficiency.
Applicants' alternative system for valve actuation replaces a conventional internal combustion engine's mechanical exhaust valve opening system with a hydro-mechanical system wherein auxiliary cam-actuated valve openings for engine braking can be inhibited or permitted by driver-initiated electrical control. Applicants' present invention preserves normal positive power operation, and incorporates certain fail-safe features in the event of electrical control failure.